This invention relates to a film embossing process of roughening a lateral edge of a thermo-plastic sheet material (hereinafter referred to as "a film" when applicable), so that the film can be readily handling during the manufacture.
In manufacturing films, embossing a lateral edge of the film, or a physical deformation is carried out in a step of the manufacture of films. This tends to improve the roll winding characteristic when the film is wound. It also facilitates the handling of the film when it is thereafter subjected to various processes.
This process is called "a film embossing process". In this process, a film is inserted between two rollers with small teeth, and the rollers abut through the film against each other, to thereby roughen a portion of the film. This process has merit that if the film embossing process is carried out with the film heated, the variation with time of the roughened portion of the film is relatively small.
When the film, having a lateral edge is roughened as described above, and wound in the form of a roll, the roughened lateral edge near the center of the roll is subjected to great pressure. This is especially true in the case where the film thus wound is stored for a long period of time after the manufacture. In such a condition a greater pressure will be applied to the roughened lateral edge near the center of the film roll. As a result, the thickness of the roughened portion of the film is decreased with time, and accordingly the object of roughening the lateral edge of the film cannot be achieved.
Furthermore, in roughening the lateral edge of a film this film embossing process, "wavy pleats" are created in the lateral edge of the film depending on a pressure applied thereto. As a result the effect of the film embossing process, which could otherwise be expected in handling the film, is often decreased. In a hot process where a film or an extruded material is heated, "wavy pleats" are significantly created.
In order to eliminate the creation of the "wavy pleats", a method has been proposed by Japanese Patent Application Publication No. 16064/1972, in which a film is locally heated to form a contracted portion immediately before the film is subjected to a film embossing process. This tends to cancel the elongation which is caused in the film embossing process. However, this method suffers in practice from difficulties because it is necessary to provide a technique for locally heating the film. Moreover, it is difficult to locally heat the film and is necessary to control the amount of contraction according to the variation of a force applied in the film embossing process. In order to exactly cancel the elongation caused during the film embossing process, and it is also necessary to change the heating temperature whenever the films to be processed are changed, because the degree of contraction depends on the kinds of film.